Powder metallurgy is often used to fabricate articles that are to be used in highly demanding applications. In such articles, it is important to achieve chemical and physical homogeneity. Articles made by casting from a melt often contain significant chemical inhomogeneities as a result of the solidification processes. They also contain physical homogeneities such as internal voids and surface cracks. The chemical and physical inhomogeneities lead to reduced physical properties or premature failure in service. Powder metallurgical production can avoid these inhomogeneities because the article is fabricated from substantially chemically homogeneous powder particles that are processed to avoid physical inhomogeneities.
To make an article by a powder metallurgy approach, powder particles are first prepared. Several powder-preparation approaches are known. In inert gas atomization, for example, the material to be made into powder is melted, and a jet of an inert gas is directed against a stream of the molten metal. Droplets of the molten metal are produced, and these droplets solidify to form the powder particles. The powder particles are graded to separate the size fractions of interest. Other atomization methods include water, oil, vacuum atomization, and centrifugal atomization, including processes such as rotating electrode, spinning cup, and rotating disk methods. However produced, the powder particles are thereafter processed to form the article. Typically, the powder particles are consolidated together by a technique such as canned extrusion, cold or hot pressing, or hot isostatic pressing. The consolidated powder is final processed by forging, machining, heat treating, and/or other appropriate operations.
The powder metallurgical approach has significant limitations as to the compositions of the powder that may be produced, and thence on the compositions of the final articles. Some alloying elements of interest cannot be practically incorporated into the melt from which the powder is made, or may be incorporated but not in the desired form or spatial distribution, due to a variety of thermodynamic and/or kinetic reasons. As a result, the compositions of powder particles that are available for processing are limited.
There is a need for an improved approach to the fabrication of articles that have the beneficial features achieved through powder metallurgical techniques, but are not limited in their compositions or physical structures in the manner of current powders. The present invention fulfills this need, and further provides related advantages.